This invention relates to an automatic transmission for an automotive vehicle wherein four forward speed power trains with an over-driving ratio and a reverse power train can be provided between the engine and the driving wheels of the vehicle.
A variety of gear trains or speed changing mechanisms with an over-driving ratio have been proposed in particular for the purpose of reducing fuel costs to promote the more efficient utilization and conservation of energy resources. According to the prior art transmissions of this type, however, one or more of the following defects can not be eliminated.
(1) Only a limited number of driving ratios can be provided so that it is difficult to obtain a sufficient number of proper driving ratios.
(2) A certain member of the gear trains rotates at an extremely high speed.
(3) The transmissions are complicated in construction. The number of parts increases.
(4) Speed change control is complicated.
For example, an automotive vehicle transmission employing an over-drive mechanism is disclosed in U.S. Pat. No. 2,725,762 patented Dec. 6, 1955 to Wayman et al. This transmission includes three friction clutches and two friction brakes. The clutch which is connected to an input shaft is additionally provided to a carrier of the third speed gear train. Although the construction of the transmission is simple, the defects (1) and (2) as specified above can not be completely eliminated.
Another transmission for an automotive vehicle is disclosed in the Japanese Patent Publication No. 51-9092, in which a gear set and a brake are added so as to provide an over-drive ratio. In this transmission, the construction thereof is simple, and setting of all desired gear ratios is relatively flexible, but still the defect (2) as above-stated can not be eliminated.